(1) Field of the Invention
The present invention relates to tunable filters. More specifically, the present invention pertains to a method and apparatus that allows tuning of a filter over a large optical band.
Narrow-band imaging is an important ingredient of observational sciences such as astronomy, remote sensing, Raman imaging, and molecular analysis. The capacity to isolate a known spectral feature allows a deep view of the universe by removing unwanted photons from the background and from the source of interest itself. Faint high redshift clusters of galaxies, emission line comparisons across extended objects, and Raman microscopic imaging are just a few examples of the compelling research relying on band-limiting filters. In most cases, fixed-interference filters are used, which often limit the observations to a few spectral features. Observing capabilities are greatly enhanced when a tunable filter is employed in place of the fixed filters.
(2) Description of Related Art
Currently available tunable filter technologies are mostly based on low-order Fabry-Perot etalons, such as those described in J. Bland-Hawthorn and D. Jones, Taurus Tunable Filter: A flexible approach to narrowband imaging, Publications Astronomical Society of Australia, vol. 15, no. 1, p. 44-49 (1998). Other available tunable filter technologies include acousto-optics and Lyot filters, such as those discussed in Joss Bland-Hawthorn and D. H. Jones, A Tunable Lyot Filter at Prime Focus: A Method for Tracing Supercluster, Proceedings SPIE, vol. 3355, p. 855-65 (2001). One drawback of the Fabry-Perot filters is that they are restricted to very narrow bandwidth, and average transmissivity. They transmit multiple inference orders and thus necessitate intermediate band blocking filters. Lyot and acousto-optics filters are promising for wide field imagers, but work in polarized light. Therefore, the Lyot and acousto-optic filters intrinsically reject at least half of the light or require two detector planes. Thus, they have a low throughput, besides being optically very complex. What is needed is a tunable filter over an optical band, with increased efficiency, without periodic orders, and without additional efficiency losses.
Although work has been done in this area, one drawback is that the systems proposed are optically complex, limited in their tunablity and efficiency. What is needed is a simpler system with an adjustable pass band and greater efficiency.